1. Field of the Invention
The present invention relates to signal processing, and more particularly, to an equalizer and a related equalizing method with a variable delay time which can reduce stub effect in high speed data communication system.
2. Description of the Prior Art
With reference to FIG. 1, a communication system 100 including a transmitting end 110 and a receiving end 120 is shown, wherein the transmitting end 110 and the receiving end 120 are coupled through a cable 115. The transmitting end 110 includes a transmitter 114, and the transmitter 114 may be disposed on a Printer circuit board (PCB) on which a connector (not shown) is also disposed. The connector is utilized for connecting the cable 115 and coupled to the transmitter 114 through a PCB trace 116. Normally, a signal can be sent from the transmitter 114 through the PCB trace 116 and the cable 115 to the receiving end 120. However, in the design of the transmitting end 110, an unnecessary PCB trace 112 may be improperly implemented on the PCB due to some considerations, which forms a stub. The stub may lead to a stub effect, which leads to signal distortion, especially in high speed data communication system (e.g. High-Definition Multimedia Interface, HDMI). The stub effect is illustrated as below.
Referring to FIG. 1, when a signal Va which intends to be transmitted from the transmitter 114 of the transmitting end 110 to the receiving end 120 passes through a stub point A, a reflected signal Vb will be further generated by the signal Va reflected upon a point B. Thus, on a point C, the signal Va and the reflected signal Vb will be superposed as a signal Vcable; however, what is actually received by the receiving end 120 will be the signal Vcable rather than the original signal Va, which leads to signal distortion (namely the stub effect). Further differences between signals Va, Vb and Vcable are illustrated in FIG. 2.
Referring to FIG. 2, as the reflected signal Vb is generated due to the reflection of the signal Va, there exists a delay time Td1 between the reflected signal Vb and the signal Va. The delay time Td1 is further relative to the length of the stub (namely the PCB trace 112). Accordingly, the signal Vcable to be transmitted through the cable 115 is therefore quite different from the original signal Va outputted from the transmitter 114 so that the receiving end 120 may acquire wrong information from the signal Vcable.
Usually, such a signal distortion due to reflection can be reduced by an equalizer at the receiving end. However, as the delay time of the reflected signal is relative to the length of the stub, the delay time becomes un-predictable. This is because the PCB trace 112 is not always consistent. In some designs the PCB trace 112 may be very short while in other designs the PCB trace 112 may be very long. Even, the PCB trace 112 may not exist in still other designs. Thus, the conventional equalizer can not effectively and properly reduce the stub effect, which may further makes the equalized signal worse than the un-equalized signal.